The present invention relates to a photographic element for the production of lithographic printing plates by the silver complex diffusion transfer process.
Lithographic printing plates consist of greasy ink receptive oleophilic image portions and ink repellent oleophobic non-image portions and the non-image portions are generally water receptive hydrophilic areas. The usual lithographic printing is carried out by feeding both water and ink to the printing plate surface to allow the image portions to receive preferentially colored ink and the non-image portions to receive preferentially water and then transferring the ink deposited on the image portions onto a substrate such as paper.
Accordingly, in order to obtain prints of good quality, it is necessary that oleophilic activity of the image portions and hydrophilic activity of the non-image portions are both strong enough so that when water and ink are applied, the image portions may receive sufficient amount of ink while the non-image portions may completely repel the ink.
Light-sensitive elements used for making lithographic printing plates utilizing the silver complex diffusion transfer process (DTR process), especially those which comprise a silver halide emulsion layer and a physical development nuclei layer provided thereon have been proposed, for example, in U.S. Pat. Nos. 3,728,114, 4,134,769, 4,160,670, 4,336,321, 4,501,811, 4,510,228 and 4,621,041. According to these techniques, exposed silver halide crystal undergoes chemical development by DTR development to become black silver, which constitutes hydrophilic non-image portions and unexposed silver halide crystal is converted to a silver complex salt by the silver halide complexing agent contained in the developer, which diffuses to the surface physical development nuclei layer and undergoes physical development in the presence of the nuclei to form the ink receptive image portions mainly composed of physically developed silver.
In these lithographic printing plates, silver images precipitated on the physical development nuclei layer provided on gelatino-silver halide emulsion layer are utilized as ink receptive image portions. Therefore, these printing plates are inferior to general lithographic printing plates such as PS plate in resistance to mechanical wear of image portions and have the defects that the image portions wear off or the ink receptivity of the image portions is gradually lost during printing.
If hardness of gelatin is enhanced or amount of the physical development nuclei is increased in order to solve the above defects, staining resistance of the printing plates much lowers. Further, in the above lithographic printing plates hydrophilicity of the non-image portions is realized mainly by hydrophilicity of the surface of gelatin and this has the defect that staining resistance is apt to be reduced by printing as compared with hydrophilized aluminum surface which is mainly used in general lithographic printing plates.
Influence of surface roughness of lithographic printing plates on printability has also been known in the field of PS plate and it has become clear that the surface roughness is also an important factor in the lithographic printing plates made utilizing the silver complex diffusion transfer process of the present invention. Especially, in the lithographic printing plates made using the silver complex diffusion transfer process, the non-image portions are composed mainly of gelatin film and although the gelatin film is inherently high in hydrophilicity, generally the surface has been designed to have high matte property (high irregularity) by matting agents used for improving printing endurance of the image portions. The inventors have studied the correlation between the matte property of the surface and staining caused by printing and have found that staining of non-image portions caused by printing can be markedly reduced by specifying the arithmetical mean deviation of profile (Ra value) of the light-sensitive element in which a matting agent is contained in an undercoat layer.
For printing, water-carrying capacity (or ink/water balance) is important and higher response is preferred in setting the proper printing conditions by changing feeding of ink and that of water. Especially, in the field where offset rotary presses are used, the etching by dampening solution is often omitted at the beginning of printing in order to prevent breaking of paper and thus, ink deposits on non-image portions. Therefore, at the beginning of printing, it is important to clear the ink and fast start up is required.
There is the field of plate making by using commercially available process camera having no reversing mirror as one application of light-sensitive elements for making lithographic printing plates using the silver complex diffusion transfer process. Hitherto, printing plates can be directly obtained by using a process camera provided for reprophotography and by using light-sensitive elements to be exposed from back side thereof and utilizing the silver complex diffusion transfer process. With recent spread of composer cameras such as for opti-copies, the above-mentioned improvements have also been demanded for improving printability of lithographic printing plates made by exposure from back side of light-sensitive elements.
Furthermore, lithographic printing plates made by exposure from back side have the restriction that compounds which increase total transmission density of respective constituting photographic layers through which light passes in exposure from back side, such as titanium oxide, cannot be used since much reduction in sensitivity is brought about although they are effective for diminishing stains caused by printing.